Preheat the oven to 400 degrees F. (200 degrees C/Gas 6)
Cut the squash in half and remove the seeds, reserving these for later. Using your pestle and mortar, bash up the coriander and chili, nice and fine, and sprinkle over the squash. Drizzle with a little oil and roast in a hot oven until nice and soft.
Heat a little oil in a large pan and fry the pancetta with half the sage leaves. Add the chestnuts and the onion and fry gently for about 15 minutes. By this time the squash will be ready, so spoon it out into the pan, leaving the skin behind. Cover with the stock and bring to the boil. Remove half the mixture and blend until smooth, then pour back into the pan.
Take the rest of the sage leaves and the reserved squash seeds and fry them in a little oil until crisp. To serve, spoon a little sour cream on top of the soup, sprinkle with a little of the crisp sage and seeds, and finish with a good drizzle of extra-virgin olive oil.